The practice, by which traders invest heavily in high-speed technology that allows them to react to market moves in fractions of a second, has come into question amid fresh claims from author Michael Lewis that it is "rigging" the market.

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Holder said the matter has “rightly received scrutiny.”

“I can confirm that we at the United States Department of Justice are investigating this practice to determine whether it violates insider trading laws,” he told appropriators. “We are determined to follow this investigation wherever the facts and the law may lead.”

Holder said he had only begun to understand the details of the trading operations and was “really getting up to speed on this."

In the trades, “even milliseconds can matter," he said.

“The concern is that people are getting an inappropriate advantage, information advantage — I guess competitive advantage — over others because of the way in which the system works,” he said.

High-frequency trading has been subject to some debate in the past, but the scrutiny reached a new level this past week. Lewis, who has published a new book on high-frequency traders, asserted this week that the stock market was effectively rigged for average investors.

Lewis says that with incredible speed on their side, high-frequency traders are able to see and react to changes in the market almost instantly, leading some critics to argue they can react to orders before they are even filled.

“The insiders are able to move faster than you,” he told “60 Minutes.”

Earlier this week, another top financial regulator said they were also examining the practice. Mary Jo White, commissioner of the Securities and Exchange Commission, told lawmakers Tuesday that there were “a number” of ongoing investigations regarding market integrity, including the impact of high-frequency trading.

Holder said that the Justice Department would coordinate with the SEC to ensure that the parallel operations did not interfere with each other.

The Commodity Futures Trading Commission is also exploring the matter, although Acting Commissioner Mark Wetjen said Thursday he did not think the markets were unfairly rigged.